Many different types of photovoltaic devices are known in the art (e.g., see U.S. Patent Document Nos. 2004/0261841, 2006/0180200, U.S. Pat. Nos. 4,335,266, 4,611,091, 6,784,361, 6,288,325, 6,631,603, and 6,123,824, the disclosures of which are incorporated by reference herein in their entireties). Examples of known photovoltaic devices include CIGS (approximately Cu(In, Ga)(Se,S)2 and/or CuInx-1GaxSe2) solar cells. CIGS films are conductive semiconductor compounds that are often referred to as an absorber or light absorbing layer(s). Generally speaking, CIGS type photovoltaic devices include, from the front (or light incident) side moving rearwardly, a front transparent cover sheet (or substrate) such as glass, a front electrode comprising a transparent conductive layer(s) (e.g., a transparent conductive oxide), a light absorption semiconductor film (e.g., CIGS), a rear electrode/contact, and a rear substrate of a material such as glass or metal foil for certain example flexible applications. In some instances, an adhesive may be provided between the front substrate and the front electrode. It is also the case in some instances that the device is provided with window layer(s) (e.g., of or including CdS, ZnO, or the like). Photovoltaic power is generated when light incident on the front side (or front substrate) of the device passes through the front electrode and is absorbed by the light absorption semiconductor film (e.g., CIGS) as is known in the art.
For example, with reference to FIG. 1, there is generally provided a schematic cross-sectional diagram illustrating various elements of a CIGS-type photovoltaic device 10. The solar cell includes a rear glass substrate (or back glass) 12. A back contact made up of a metal layer, such as, for example, molybdenum (Mo) 14 is typically deposited on the rear glass substrate 12. The first active region of the device 10 comprises a semiconductor layer 16 which is typically a p-type copper indium/gallium diselenide (CIGS) which may be deposited by coevaporation. A thin “window” layer of n-type compound semiconductor 18, typically comprising cadmium sulfide (CdS), may then be wet deposited on CIGS layer 16. A front electrode 20 (e.g., of conductive zinc oxide) is deposited on the CdS layer 18 and acts as a transparent front contact for the photovoltaic device 10. The device 10 may be completed by including a series of front face contacts (not shown) in the form of, for example, a metal grid on top of the transparent front contact 20 to facilitate the extraction of generated electrons, and a front glass substrate 22. A large solar cell may also be divided into a number or smaller cells by means of scribes, such as, for example, laser or mechanical scribes or the like, traditionally referred to as P1, P2 and P3, which allow individual cells to be connected in series.
As noted above, a metal such as Mo may be used as the rear electrode (or back contact) 14 of a photovoltaic device, such as, for example, a CIGS solar cell 10, to extract positive charges (holes) generated in the CIGS semiconductor absorber 16. In certain instances, the Mo rear electrode 14 may be sputter-deposited using, for example, direct-current magnetron sputtering, onto the back glass substrate 12. However, using Mo alone as the material for the back contact 14 of the solar cell 10 suffers from certain disadvantages. For example, Mo has a relatively high material cost and sometimes suffers from the problem of delamination from the back glass substrate. In addition, Mo typically exhibits relatively low conductivity and suffers from a relatively slow deposition rate, and thus causes correspondingly low production line throughput. As a result, using Mo as the material for the back contact accounts for a substantial portion of the total device cost. Moreover, a frequent desire for a back contact for CIGS type solar cells is to achieve a sheet resistance (Rs) on the order of less than or equal to about 1 ohm/square. In order for a pure Mo back contact to alone meet this requirement, the Mo thickness must typically range from 300-900 nm, depending upon the CIGS deposition method. In other words, the thicker the film, the lower its sheet resistance. This results in the back contact making up about 15-25% of the total photovoltaic module cost.
Attempts have been made to use other types of metals as the sole material to form the back contact. These attempts have resulted in limited success with only a few other materials, such as, for example, tungsten (W), tantalum (Ta) and niobium (Nb), exhibiting sufficient compatibility with the CIGS absorber, e.g., not reacting with the CIGS absorber. However, these alternatives result in a lower CIGS device efficiency as compared to devices using Mo as the material for the back contact.
In certain example embodiments of this invention, a substantial portion of the Mo back contact is substituted with copper (Cu), which is a less expensive and a more conductive alternative. It has been found that substituting Cu for a portion of the Mo of the back contact does not significantly compromise cell performance in a CIGS cell. The many advantages of using Cu as the bulk (or at least part) of the back contact include, for example, lower cost—Cu sputtering targets are typically at least two times less expensive than using Mo sputtering targets. Additionally, the sputter deposition rates of Cu are typically at least two times greater than that of Mo. Another advantage of using Cu in the back contact is that sputtered Cu has greater conductivity as compared to sputtered Mo. The Cu portion of the rear electrode/contact is preferably separated from the CIGS absorber by at least a layer of or including Mo.
When a Cu layer is provided in the rear electrode, a Mo layer is provided as part of the rear electrode/contact between the Cu layer and the CIGS absorber in order to prevent or reduce Se penetration into the Cu so that significant Se from the CIGS does not react with the Cu during the high-temperature selenization process. One or both of the Cu and/or Mo layers may be reflective layers in certain example embodiments. In example embodiments of this invention, in order to allow for a reduced thickness of the conductive Mo inclusive layer, a selenium blocking layer(s) is located between the conductive Cu based layer and the conductive Mo inclusive layer in the rear electrode/contact. The selenium blocking layer may contain, for example, Cu and at least one more element selected so that its enthalpy of formation is smaller compared to copper selenide so that there is no or substantially no replacement of the portion of the compound material with selenium. The selenium blocking layer may be of or include CuO, Cu2O, CuN2O6, CuN2 and/or MoN in certain example embodiments of this invention.
In certain example embodiments of this invention, there is provided a photovoltaic device, comprising: a front substrate; a semiconductor absorber film; a rear contact comprising a first conductive layer comprising copper, a second conductive layer comprising molybdenum, and a selenium blocking layer located between at least the first conductive layer comprising copper and the second conductive layer comprising molybdenum; and a rear substrate; wherein the first conductive layer comprising copper is located between at least the rear substrate and the selenium blocking layer, and wherein the semiconductor absorber film is located between at least the back contact and the front substrate.
In certain example embodiments of this invention, there is provided a method of making a coated article for use in a photovoltaic device, the method comprising: providing a substrate; depositing a conductive layer comprising Cu over at least said substrate; depositing a selenium blocking layer on the substrate over at least the conductive layer comprising Cu; depositing a conductive layer comprising Mo on the substrate over at least the first conductive layer and the selenium blocking layer; and forming a CIGS absorber film over at least said second conductive layer.
In certain example embodiments of this invention, there is provided a photovoltaic device comprising: a front substrate (e.g., glass substrate); a semiconductor absorber film (e.g., CIGS inclusive); a rear contact comprising a conductive layer comprising copper, a layer comprising molybdenum which may or may not be conductive, and a layer comprising an oxide and/or nitride of copper located between at least the conductive layer comprising copper and the layer comprising molybdenum. The device further includes a rear substrate (e.g., glass or foil substrate), wherein the conductive layer comprising copper is located between at least the rear substrate and the layer comprising the oxide and/or nitride of copper, and wherein the semiconductor absorber film is located between at least the back contact and the front substrate.
These and other example embodiments and example advantages are described herein with respect to certain example embodiments and with reference to the following drawings in which like reference numerals refer to like elements, and wherein: